Glass transformation, heat capacity and structure of AsxSe1-x glasses studied by modulated temperature differential scanning calorimetry experiments

The recent novel temperature modulated differential scanning calorimetry (MDSC(TM) TA Instruments Inc. USA) technique has been applied to the measurement of the thermal properties of chalcogenide glasses of the As-Se system in the glass transition region. All samples in this work were given the same thermal history by heating to a temperature above the glass transition, equilibrating and then cooling at a rate of 5 degrees C min(-1) to a temperature of 20 degrees C. The reversing and non-reversing heat flows through the glass transformation region during both heating and cooling schedules were measured and the values of the parameters T-g, Delta H, C-p and Delta C-p, which characterize the thermal events in the glass transition region, were determined. The ability to determine the reversible heat flow in MDSC enables an accurate measurement to be made of the true heat capacity (that normally associated with reversible heat flow). This could not be done hitherto in conventional thermal analysis where the detected heat flow is the total heat flow, i.e. the sum of reversing and non-reversing heat flows. The structurally determined parameters T-g, Delta H, C-p and Delta C-p reveal extrema when the As-Se glass system reaches the average coordination number [r] = 2.40 at 40 at.% As, which correspond to the stoichiometric composition As2Se3. We also observed local extrema in the composition dependence of the above thermal parameters at 16.6 and 28.5 at.% As, which correspond to the stoichiometric compositions As2Se10 and As2Se5 with average coordination numbers 2.52 and 2.57, respectively. No such clear local extrema below the 40 at.% As composition could be observed previously in thermal analysis. We compare our MDSC results with previously published works on glass transition in As-Se glasses and discuss the results in terms of recent structural models for chalcogenide glasses.